Imaging devices comprising an array of pixels of various types are known and may or may not directly allow TDI mode of operation.
Charge coupled image sensors (also known as charge coupled devices (CCDs)) form one type of known imaging device. A CCD device operates in the following way.
1. Charge is accumulated in a depletion region created by an applied voltage. For each pixel the depletion has a potential well shape and constrains electrons under one electrode gate to remain within the semiconductor substrate.
2. Voltage is applied as pulse to the electrode gate of the CCD device to clock each charge to an adjacent cell. The charge remains inside the semiconductor substrate and is clocked through, pixel by pixel, to a common output. During this process no additional charge can be accumulated.
Another type of imaging device is the Active-pixel Semiconductor Imaging Device (ASID) described in International application publication number WO95/33332 and incorporated herein by reference. The ASID comprises an array of pixels including a semiconductor substrate having an array of detector cells and a further array of pixel circuits. The detector cells generate charge in response to incident radiation. Each pixel circuit associated with a respective detector cell accumulates charge resulting from plural radiation hits of the associated detector cell. At determined times the charge from the pixel circuits can be read out and used to generate an image based on the analogue charge values stored in each pixel circuit.
A further type of imaging devices are photon counting devices (PCD) described in International application publication number WO98/16853 and incorporated herein by reference. The PCD comprises an array of pixels including a semiconductor substrate having an array of detector cells and a further array of pixel circuits. The detector cells generate charge in response to incident radiation. Each pixel circuit comprises a discriminator registering radiation hits of a preferred energy range for a respective detector cell, and an n-bit digital counter and an n-bit loadable shift-register.
The PCD works in the following way.
1. The number of registered radiation hits in the associated detector cell is counted into the n-bit digital counter.
2. In response to an external request the value of the counter is loaded into the n-bit shift-register and the value of the counter is cleared.
3. All shift-registers of the entire sensor are chained and the information is clocked out in a serial fashion. Steps 1 and 3 can be executed simultaneously.
Of the afore-mentioned imaging techniques only CCD devices have been used in a TDI mode of operation since charge-shifting is an inherent feature of CCD technology.
Referring to FIG. 9, there will now be described a TDI CCD detector and the operation of such a detector.
Since CCDs are sensitive only to visible light, a scintillator 80, such as a phosphor, for converting high energy radiation, such as X-rays 82, into visible light must be placed in front of the CCD device 84. The scintillator 80 is generally composed of a strip of phosphor material, and is coupled to a fibre optic taper 86, which is bonded to the active region 88 of the CCD device 84. Three fibre optic tapers 86 respectively bonded to three CCD devices 84 are illustrated in FIG. 9. The fibre optic tapers 86 de-magnify light collection to provide space at the taper outputs to accommodate the outer non-active regions of the CCD devices 84, yet still maintain an acceptable light collection efficiency.
In a TDI mode, image acquisition takes place by scanning a continuously active X-ray beam across an object, such as a breast for mammography applications, and correspondingly moving the detector across the object, typically at the same scanning speed as the X-ray beam. Charge collected in CCD pixels is shifted down the columns of the CCD devices 84 at a rate equal to, but in an opposite direction to, the scanning X-ray beam. The charge packets collected at the end of the CCD columns have effectively remained stationary with respect to a given path of X-rays through the object, and they comprise the integral of several charge packets in the CCD column. This is because the sensor remains active at all times and the total pixel charge on the image comes from integrating charge from multiple pixel cells as the total charge is clocked along one column. The collected charge packets are read out and typically digitised for processing into suitable images.
TDI mode imaging advantageously provides good total energy levels of imaging radiation and correspondingly good stability, yet from relatively low overall X-ray illumination. Thus, providing good imaging whilst reducing the inherent danger from high power X-ray illumination.
However, CCD imagers require indirect conversion of X-rays to light, and thus have limited absolute sensitivity and resolution. Direct radiation to electron converting substrates cannot be connected to a CCD because the small pixel charge capacity of CCDs saturates at very low doses of radiation, and cannot cope with the large currents provided by direct conversion.
The present invention seeks to mitigate the problems described in the prior art and embodiments provide a TDI imaging device not using a CCD.